Field of the Invention
The present invention relates to the field of digital signal receiving and processing systems and is intended for identification and compensation for inversion of the input bit stream in decoding LDPC (low density parity check) codes.
Description of the Related Art
The problem of identification and compensation for inversion of the input bit stream is important for communication radio systems and wireless data transmission systems.
The cause of phase inversion for the received signal with phase or amplitude-phase modulation can be jumps in phase of a phase synchronization system, based on the Costas loop, in the process of coherent demodulation of the received signal when signal-to-noise ratio is not very good.
The most efficient way to overcome non-controlled phase jumps is relative (differential) modulation methods, for example, DPSK. However in some cases, a transition from phase modulation to relative (differential) modulation is impossible. In particular, such a situation can be faced in modernizing some existing communications systems.
During a transition to iterative decoding methods, for example, LDPC (low density parity check) codes, inversion of the bit stream results in generating a large error burst/cluster in the code word, and the error correcting capability of the code may become insufficient to correct the errors. All the following code words until the next phase jump will be inverted and cannot be correctly decoded. In addition, in this case the decoder will try to perform the whole range of the permitted decoding iterations, which considerably increase system computational load.
For this purpose, the problem of identification for the fact of inversion of the bit stream is of special interest, as well as the development of its compensational methods.
There are known methods and devices of decoding LDPC codes described, for example, in U.S. Pat. No. 7,281,192, U.S. Pat. No. 7,475,103, U.S. Pat. No. 8,255,758, U.S. Pat. No. 8,381,079, U.S. Pat. No. 8,489,957, U.S. Pat. No. 8,601,352, U.S. Pat. No. 8,689,092, US Patent Publication No. 2009/0070659, US Patent Publication No. 2009/0249159, US Patent Publication No. 2014/0089766, and so on.
Also, there are known technical solutions providing identification of inversion of the input bit stream: U.S. Pat. No. 7,701,391, U.S. Pat. No. 8,255,758, US Patent Publication No. 2011/0200059, EP1324530, U.S. Pat. No. 7,221,292, U.S. Pat. No. 6,563,745, U.S. Pat. No. 7,549,011 and so on.
In U.S. Pat. No. 7,701,391, a method of coding and decoding GPS signals with LDPC codes is described. The device implementing the claimed method includes an LDPC decoder, an inversion detector/finder, and a buffer. However, the technical implementation of the inversion detector unit (numbered 510 in the document) has not disclosed in the patent,
In U.S. Pat. No. 8,255,758 a method of fighting against inversion of the limited bit number is proposed when there is no successful decoding for the pre-set iteration number. A drawback of this method is a necessity of performing a great number of iterations to detect the fact of bit inversion of the code word.
US Patent Publication No. 2011/0200059 describes a method and a device providing a search for inverted bits in the received and preceded data burst with the help of an inversion indicator.
EP1324530 includes a description of a data transmission system using bit inversion and comprising an inversion detection unit, and a data inversion unit. But to detect the fact of inversion, a special inversion detection unit is used, which complicates the system implementation.
A bit inversion system is also described in U.S. Pat. No. 7,221,292 that is based on a comparison of the number of distinguishing bits in two neighboring words of the data package. However, this technical solution is based on a specific data configuration in packages and hence has a limited use.
US Patent Publication No. 2007/0118790 describes a method and device to stop iterative decoding in mobile communication systems. The method includes iterative decoding of the received signal and calculating a stop condition. A corresponding device includes an iterative decoder and units providing a calculation of the condition of decoding stop. But this method and device cannot be used in decoding LDPC codes.
The purpose of the claimed solution is to eliminate drawbacks of known technical solutions.